This invention relates to improving the safety, reliability and performance of nuclear reactors. More specifically, the invention relates to a method and apparatus for improving conditions in the primary system of light water reactors by removing impurities that may otherwise become activated in the reactor core and deposit on surfaces within the primary circuit, leading to component degradation and increased dose rates. The invention is applicable to pressurized water reactors (PWRs) and boiling water reactors (BWRs).
PWR nuclear power reactors use recirculating subcooled water in the reactor coolant system (RCS) to remove energy produced by fission in the core. The recirculating water flows upward under pressure through the core and then to the “primary side” of one or more steam generators, wherein the energy is transferred through the steam generator tubes to the secondary side of the steam generators where water is boiled to produce saturated or superheated steam. Most of the steam produced in the steam generator is directed to a turbine generator to produce electricity. Some steam is used to reheat steam in the secondary cycle, drive steam turbine driven pumps, or preheat feedwater that is fed to the steam generators. In some plants, steam may be used for other purpose such as seawater desalinization.
BWR nuclear power reactors use recirculating water to remove energy produced by fission in the core but unlike in PWRs, pool boiling occurs within the core of the reactor. To maintain favorable heat transfer and control of the fission process in BWRs, one or more recirculation loops are used to forcibly circulate liquid water upward through the core. Favorable conditions include: (1) enhanced convective and boiling heat flux at the higher velocities produced by recirculation, and (2) a higher liquid water fraction which increases moderation of neutrons. Steam generated in the core is separated from the recirculating steam-water mixture produced in the core and directed to a turbine generator to produce electricity, secondary cycle steam reheaters, steam turbine driven pumps, or to feedwater heaters to pre-heat recycled feedwater. The liquid phase water that exits the core with the steam is separated from the steam and is pumped back into the lower end of the core with the recirculation pumps. The recirculation pumps may be motor driven centrifugal pumps or a combination of jet pumps and centrifugal pumps.
Nuclear power reactors generate heat in their cores by fission of fissile materials such as U-235 or Pu-239. The cores may also contain fertile materials such as Th-233, which can be converted to fissionable species by irradiation in the core. The concentration of fissile material in PWR or BWR fuel is typically enriched over what is found in nature. The enrichment is typically to 2 to 20%, but it may be much higher. The balance of the fuel is typically naturally occurring non-fissile material (e.g., U-238). In light water reactors, the neutrons produced by the fission process are “moderated” by the water. Moderation lowers the energy of the neutrons and renders them more likely to promote desirable fission chain reactions with the fuel.
The chemical form of the fuel in most reactors is solid uranium oxide or a mixture of uranium oxide and plutonium oxide, but other forms that may be used include uranium or plutonium solid metal alloys. In general, the oxide fuel is formed into cylindrical pellets, which are stacked in fuel rods within “cladding” and further grouped in fuel rod assemblies. Most cladding is fabricated from zirconium alloys owing to the transparency of zirconium to neutrons, known as low neutron cross section, as well as the good corrosion resistance of zirconium alloys. A typical PWR may contain about 200 fuel assembles, each of which contains about 250 fuel rods (or pins) that are 3 to 5 meters in length. A typical BWR may contain 600 to 800 fuel assemblies, each of which typically contains 60 to 100 fuel rods (or pins) that are 3 to 4 meters in length. BWR fuel is also typically housed in removable “channels” which are elongated square tubes. The main purpose of the channel is to prevent cross flow of water and steam from assembly to assembly which further serves to ensure favorable thermal hydraulics, heat transfer and control of the fission process. In PWRs, cross flow of water from assembly to assembly is not avoided; therefore, fuel rods are not channeled but distributed in an open square or triangular pitch array.